Field of the Invention
This invention relates to bonding tool tips and more particularly to dissipative ceramic bonding tips for bonding electrical connections.
Description of the Prior Art
Integrated circuits are typically attached to a lead frame, and individual leads are connected to individual bond pads on the integrated circuit with wire. The wire is fed through a tubular bonding tool tip having a bonding pad at the output end. These tips are called capillary tips. An electrical discharge at the bonding tool tip supplied by a separate EFO (electronic flame off) device melts a bit of the wire, forming a bonding ball. Other bonding tools do not have the center tube, but have a feed hole or other feature for feeding the wire along, as needed. Some bonding tips have no such wire arrangement, as the wire is supplied, as in magnetic disk recording devices, where the wire is insulated and bonded to a magnetic head and then to a flexible wire circuit.
When the bonding tip is on the integrated circuit die side of the wire connection, the wire will have a ball formed on the end of the wire, as above, before reaching the next die bonding pad. The ball then makes intimate contact with the film formed on the die pad on the integrated circuit. The bonding tip is then moved from the integrated circuit die pad, with gold wire being fed out as the tool is moved, onto the bond pad on the lead frame, and then scrubbed laterally by an ultrasonic transducer. Pressure from the bonding tool tip and the transducer, and capillary action, xe2x80x98flowsxe2x80x99 the wire onto the bonding pad where molecular bonds produce a reliable electrical and mechanical connection.
Bonding tool tips must be sufficiently hard to prevent deformation under pressure, and mechanically durable so that many bonds can be made before replacement. Prior art bonding tool tips were made of aluminum oxide, which is an insulator, but provides the wearability to form thousands of bonding connections. Bonding tool tips must also be electrically designed to produce a reliable electrical contact, yet prevent electrostatic discharge damage to the part being bonded. Certain prior art devices have a one or more volt emission when the tip makes bonding contact. This could present a problem, as a one volt static discharge could generate a 20 milliamp current to flow, which, in certain instances, could cause the integrated circuit to fail due to this unwanted current.
U.S. Pat. No. 5,816,472 to Linn describes a durable alumina bonding tool xe2x80x9cwithout electrically conductive metallic binders.xe2x80x9d U.S. Pat. No. 5,616,257 to Harada describes covering the bonding tool electrode with an insulating cap or covering xe2x80x9cmade of a ceramic materialxe2x80x9d to produce a large electrostatic discharge that creates bonding balls of stable diameter. U.S. Pat. No. 5,280,979 to Poli describes a vacuum wafer-handling tool having a ceramic coating xe2x80x9cmade with a controlled conductivityxe2x80x9d to prevent a large electrostatic discharge.
Electrically, dissipative ceramic bonding tips for bonding electrical connections to bonding pads on electrical devices are disclosed. In accordance with the principles of the present invention, to avoid damaging delicate electronic devices by any electrostatic discharge, a bonding tool tip must conduct electricity at a rate sufficient to prevent charge buildup, but not at so high a rate as to overload the device being bonded. In other words, it is desirable for the bonding tip to discharge slowly. The tip needs to discharge to avoid a sudden surge of current that could damage the part being bonded. For best results, a resistance in the tip assembly itself should range from 105 to 1012 ohms. The tools must also have specific mechanical properties to function satisfactorily. The high stiffness and high abrasion resistance requirements have limited the possible material to ceramics (electrical non-conductors) or metals, such as tungsten carbide (electrical conductor).
In the present invention, bonding tool tips with the desired electrical conduction can be made with three different configurations.
First, the tools can be made from a uniform extrinsic semiconducting material which has dopant atoms in the appropriate concentration and valence states to produce sufficient mobile charge carrier densities (unbound electrons or holes) which will result in electrical conduction in the desired range. For example, polycrystalline silicon carbide uniformly doped with boron.
Second, the tools can be made by forming a thin layer of a highly doped semiconductor on an insulating core. In this case the core provides the mechanical stiffness and the semiconductor surface layer provides abrasion resistance and provides a charge carrier path from the tip to mount which will permit dissipation of electrostatic charge at an acceptable rate. For example, a diamond tip wedge that is ion implanted with boron.
Third, the tools can be made by forming a lightly doped semiconductor layer on a conducting core. The conducting core provides the mechanical stiffness and the semiconductor layer provides abrasion resistance and provides a charge carrier path from the tip to conducting core, which is electrically connected to the mount. The doping level is chosen to produce conductivity through the layer which will permit dissipation of electrostatic charge at an acceptable rate. For example, cobalt bonded tungsten carbide coated with titanium nitride carbide.